Port Length
The port length required to tune a volume of air to a specific frequency can be calculated
by using the following equation: Lv =
(23562.5*Dv^2*Np/(Fb^2*Vb))(k*Dv)
where,
Dv = port diameter (cm)
Fb = tuning frequency (Hz)
Vb = net volume (litres)
Lv = length of each port (cm)
Np = number of ports
k = end correction (normally 0.732)
The value for k, the end correction, can be finetuned by using the
following values to derive the appropriate end correction figure for each end of the port,
then adding them together
Flanged End: 0.425
Free End: 0.307
e.g. if both ends were flanged,
k = 0.425 + 0.425 = 0.850
if one flanged, one free,
k = 0.425 + 0.307 = 0.732
if both ends were free,
k = 0.307 + 0.307 = 0.614
Normally, k=0.732 is assumed
In practice, it's best to use ports that are slightly longer than predicted by
the above equations, then adjust their length until the correct tuning is achieved. It is
much easier to shorten a port than to lengthen it!
Minimum Port Diameter
To calculate the minimum diameter of the port required to prevent port noises, you will
also need to know the following:
Xmax = maximum linear displacement (mm)
Dia = Effective diameter of driver (cm)
Np = number of ports
Calculate the minimum port diameter from the following equations:
Sd = pi*(Dia/100)^2/4
Vd = Sd*Xmax/1000
Dmin = 100*(20.3*(Vd^2/Fb)^0.25)/Np^.5
where,
Dmin = minimum port diameter (cm)
Note: You CAN use ports that have a smaller diameter than that given by the
equation above, especially if the ports are flared at both ends. However, at higher
volumes, you may notice some port noise caused by the air rushing through the ports.
Slot Ports
If you wish to use a slot port, first determine the
diameter of a round port that has the same crosssectional area as the slot.
The following equation can be used to do this:
Dv' = 2*((W*H)/pi)^0.5 where, Dv'
= diameter of equivalent round port
W = width of slot
H = height of slot
Once you've calculated Dv', you can use it in the equation
for Lv above to determine the required length of the slot port to tune the
enclosure to the required frequency (Fb).
